fontconfig: add support for aliases (#524628)

nixos/modules/config/fonts/fontconfig.nix

@@ -127,6 +127,38 @@ let
       </fontconfig>
     '';
 
+  # user defined font aliases
+  # priority 53
+  aliases =
+    let
+      mkFontBlock =
+        key: fonts:
+        lib.optionalString ((builtins.length fonts) > 0) ''
+          <${key}>
+          ${lib.concatMapStrings (font: "<family>${font}</family>") fonts}
+          </${key}>
+        '';
+
+      mkAliasBlock = family: opts: ''
+        <alias binding="${opts.binding}">
+          <family>${family}</family>
+          ${mkFontBlock "prefer" opts.prefer}
+          ${mkFontBlock "accept" opts.accept}
+          ${mkFontBlock "default" opts.default}
+        </alias>
+      '';
+    in
+    pkgs.writeText "fc-53-user-aliases.conf" ''
+      <?xml version='1.0'?>
+      <!DOCTYPE fontconfig SYSTEM 'urn:fontconfig:fonts.dtd'>
+      <fontconfig>
+
+        <!-- User defined aliases -->
+        ${lib.concatStrings (lib.mapAttrsToList mkAliasBlock cfg.aliases)}
+
+      </fontconfig>
+    '';
+
   # bitmap font options
   # priority 53
   rejectBitmaps = pkgs.writeText "fc-53-no-bitmaps.conf" ''
@@ -245,6 +277,9 @@ let
         # 53-no-bitmaps.conf
         ln -s ${rejectBitmaps} $dst/53-no-bitmaps.conf
 
+        # 53-user-aliases.conf
+        ln -s ${aliases} $dst/53-user-aliases.conf
+
         ${lib.optionalString (!cfg.allowType1) ''
           # 53-nixos-reject-type1.conf
           ln -s ${rejectType1} $dst/53-nixos-reject-type1.conf
@@ -522,6 +557,69 @@ in
           description = "Use embedded bitmaps in fonts like Calibri.";
         };
 
+        aliases = lib.mkOption {
+          type = lib.types.attrsOf (
+            lib.types.submodule {
+              options = {
+                binding = lib.mkOption {
+                  type = lib.types.enum [
+                    "same"
+                    "weak"
+                    "strong"
+                  ];
+                  default = "same";
+                  description = ''
+                    Binding precedence for this font family. See
+                    fontconfig "Font Matching" section for details.
+                  '';
+                };
+
+                prefer = lib.mkOption {
+                  type = lib.types.listOf lib.types.str;
+                  default = [ ];
+                  description = ''
+                    Fonts whose glyphs are chosen preferentially prior
+                    to fonts which match the alias family.
+                  '';
+                };
+
+                accept = lib.mkOption {
+                  type = lib.types.listOf lib.types.str;
+                  default = [ ];
+                  description = ''
+                    Fonts that are chosen if none of the preferred
+                    fonts, nor the alias family could provide the
+                    desired glyph.
+                  '';
+                };
+
+                default = lib.mkOption {
+                  type = lib.types.listOf lib.types.str;
+                  default = [ ];
+                  description = ''
+                    Last chance fallback fonts which are chosen by
+                    default if none of the other options could
+                    provide the desired glyph.
+                  '';
+                };
+              };
+            }
+          );
+          default = { };
+          example = lib.literalExpression ''
+            {
+                # use FreeSans for Greek symbols missing in Helvetica
+                "Helvetica" = {
+                    default = [ "FreeSans" ];
+                };
+            };
+          '';
+          description = ''
+            Font aliases that can substitute preferential fonts,
+            or specify custom fallback fonts.
+          '';
+        };
+
       };
 
     };
@@ -557,6 +655,9 @@ in
         # 52-nixos-default-fonts.conf
         r ${defaultFontsConf},
 
+        # 53-user-aliases.conf
+        r ${aliases},
+
         # 53-no-bitmaps.conf
         r ${rejectBitmaps},